The invention relates to the correction of amplitude nonlinearity in a television transmitter and more particularly to a selective efficiency video frequency nonlinearity correction device.
The purpose of the amplitude correction device of the invention is to correct the low frequency nonlinearities introduced in the power stages of a television transmitter.
The principle of such correction is to introduce in the video frequency signal before intermediate frequency modulation and predistortion in the low part of the video frequency spectrum, complementary to that which is generated in the high frequency power stages.
To provide this precorrection, devices are known which establish the characteristic of reverse amplitude to that of the output stage by means of a threshold correction circuit, the correction being achieved by means of six to eight adjustment parameters for the definition of the precorrection characteristic to be sufficient: three to four action thresholds are for example fixed, associated with three or four slopes corresponding to the efficiency of the correction introduced at the threshold point. This precorrector circuit is inserted before intermediate frequency modulation into the video signal processing chain at a point where the amplitude of this signal is well defined, i.e. after alignment and automatic gain control.
This solution has disadvantages: the adjustments of the correction thresholds and slopes between two correction thresholds interfere with each other. In addition, the efficiency of the correction is constant whatever the frequency of the side modulation band, whereas, since image modulation takes place as an attenuated side band, distortion in the double band zone is twice of that similar distortion in the single band.
Other correction circuits may be provided in a television system for precorrecting the distortion so that the signal from the high frequency stages has suitable linearity. These systems may for example be threshold precorrectors acting at the intermediate frequency level (used especially when the video frequency signal is not available, for example in television retransmitters), differential gain correctors using an order 3 nonlinearity generator, or premodulation correction circuits.
Like the threshold correction circuit described above, none of these correction circuits are selective in frequency, and it is therefore not possible to modulate separately the amplitude/low frequency amplitude characteristics and at the frequency of the chrominance sub-carrier so to suitably precorrect the transmitted signal. Moreover, a correction device of the type using an order 3 nonlinearity generator only allows progressive amplitude correction: expansion from white to black for negative modulation or expansion from black to white for positive modulation.
For the transmission of the television image signal, an amplitude modulation transmission system of the type described in the French Pat. application No. 78 27255, in the name of the present Applicants, provides amplification through a device having two parallel channels, one for amplification of a reduced carrier signal with a class AB biased amplifier, and the other for the amplification of a pure carrier signal with a class C amplifier, the signals obtained being combined so as to obtain the amplified signal to be transmitted.
The class AB amplifier introduces distortion at low levels because of the nonlinearity of its transfer characteristic, its gain being higher for low levels. These low levels of the crossed-modulation modulated carrier, with reduced carrier, correspond to the mean levels of the video frequency signal. This distortion causes an expansion of the mean gray level not only with respect to white but also with respect to black. To precorrect such an amplitude distortion, it is necessary to be able to attenuate only the grays, without affecting the amplification of the whites or blacks.
The present invention is concerned with a correction device for resolving the prior art problems of expansion of the intermediate levels in a television transmitter by providing the solution of a frequency selective video frequency corrector circuit which is easy to adjust and which further allows the differential gain not to be affected. This correction circuit may be adapted for correcting any amplitude distortion, which might affect the video frequency signal at the output of the high frequency stages in the absence of such a circuit, whatever the type of modulation used in the transmitter.